This invention relates to a laminated structure comprising a non-crosslinked foam and polybutene-1 as a substrate. More specifically, it relates to a laminated structure comprising a non-crosslinked foam as a substrate which has excellent flexibility, sealability, heat resistance, heat-resistant creep characteristics and hygienic property and is suitable as a packing material for bottle closures.
In recent years, cans have been superseded to some extent by bottles as containers for carbonated drinks. It is also desired to replace ampoules for drugs by bottles equipped with screw caps in order to avoid inclusion of glass fragments at the time of opening. Since the caps of these bottles naturally require sealability, cap liners of thermoplastic resins, especially foams of high-pressure low-density polyethylene, are frequently used.
Containers for holding drugs or drinks are usually subjected to a high-temperature sterilizing treatment after they are filled with a liquid. This sterilizing temperature has recently tended to increase in order to shorten the time and increase productivity, and now high temperatures of 100.degree. C. or more are mainly used in the sterilizing treatment.
The high-pressure low-density polyethylene foam mainly used in the past has a disadvantage that it has a low heat-resistant temperature, and cannot be sterilized at high temperatures of 100.degree. C. or more. Polypropylene has better heat resistance, but a uniform foam is extremely difficult to obtain from the polypropylene alone. In an attempt to overcome this defect of the polypropylene, Japanese Patent Publication No. 40167/1981 discloses the addition of 10 to 70% by weight of polybutene to polypropylene. The foam described in this patent document has excellent heat resistance, but since it has a high polypropylene content, it lacks flexibility and has poor sealability with respect to a bottle mouth. Accordingly, this foam is unsuitable as a packing material.
Polybutene-1 is a resin having flexibility, heat resistance and heat-resistant creep characteristics. A foam having a uniform closed cellular structure is very difficult to produce from polybutene-1 alone. Addition of a small amount of polypropylene to it improves the viscoelasticity and therefore foamability of the polybutene-1. However, since the range of temperatures suitable for foaming the resulting blend is very narrow, it is very difficult to expand and mold the blend continuously. For continuous expansion and molding, a special screw or die must be used.
It is an object of this invention to provide a laminated structure comprising a non-crosslinked foam layer of a polymer composed mainly of polybutene-1 as a substrate.
Another object of this invention is to provide a laminated structure having excellent flexibility, sealability, heat resistance, heat-resistant creep characteristics and hygienic property.
Still another object of this invention is to provide a laminated structure comprising a non-crosslinked foam as a substrate and being suitable as a packing material for bottle closures or the like.
Other objects of this invention along with its advantages will become apparent from the following description.